The present invention relates to a heat-sensitive recording material wherein a developed black color image shows absorption in the near infrared region and is excellent in moisture resistance, resistance to plasticizer, oil resistance and light fastness. More specifically, the present invention relates to a heat-sensitive recording material comprising, as a chromogenic dye-precursor,
(A) a mixture comprising at least two divinyl phthalide compounds represented by the formula (I): ##STR3## wherein R.sup.1, R.sup.2 represent respectively alkyl group of 1 to 6 carbon atoms, alkyl group of 1 to 4 carbon atoms having alkoxy group of 1 to 4 carbon atoms or cycloalkyl group of 5 to 7 carbon atoms, X.sup.1, X.sup.2 represent respectively hydrogen atom, alkyl group of 1 to 8 carbon atoms, alkoxy group of 1 to 8 carbon atoms, alkoxy group of 1 to 4 carbon atoms having alkoxy group of 1 to 4 carbon atoms, cycloalkoxy group of 5 to 7 carbon atoms, alkenyloxy group of 3 to 8 carbon atoms, benzyloxy group, substituted benzyloxy group, furfuryloxy group, tetrahydrofurfuryloxy group, phenyl group, substituted phenyl group, phenoxy group, substituted phenoxy group, fluorine atom, chlorine atom or bromine atom, and m and n represent 0 or integer of 1 to 4, n+m=4 and R.sup.1 and R.sup.2 may join together to form a heterocyclic ring, or
(B) a mixture of the divinyl phathalide compound(s) represented by the formula (I) and at least one compound selected from the compounds represented by the formulas (II), (III) and (IV). ##STR4## wherein R.sup.3 -R.sup.14 represent alkyl group of 1 to 6 carbon atoms, alkyl group of 1 to 4 carbon atoms having alkoxy group of 1 to 4 carbon atoms, cycloalkyl group of 5 to 7 carbon atoms, phenyl group, substituted phenyl group or tetrahydrofurfuryl group, p and q represent 0 or integer of 1 to 4, p+q=4 and X.sup.3 represents hydrogen atom, alkyl group of 1 to 4 carbon atoms or chlorine atom, X.sup.4, X.sup.5 represent hydrogen atom, alkyl group of 1 to 4 carbon atoms, chlorine atoms, fluorine atom or trifluoromethyl group, and each of R.sup.3 and R.sup.4, R.sup.5 and R.sup.6, R.sup.7 and R.sup.8, R.sup.9 and R.sup.10, R.sup.11 and R.sup.12, and R.sup.13 and R.sup.14 may join together to form a heterocyclic ring.
Heat-sensitive recording paper sheets using so-called leuco dyes have generally been used as recording paper for facsimiles and various printers, POS (point of sales) labels, railway tickets, etc. and the demands for the same have increased rapidly, since they have the appearance of ordinary paper, require no fixing because of primary color development and enable high speed recording with no noises, and the devices using them are small in the size, reduced in the weight, inexpensive and are free from maintainance work.
It is important for such heat-sensitive recording paper sheet that it is free from background coloration and excellent in color development sensitivity, color development density, and light fastness and storage stability (moisture resistance, resistance to plasticizer, oil resistance, etc.) of developed color image. It is also important that a heat-sensitive recording paper wherein developed color image shows absorption at the near infrared region has the same properties as above, but no satisfactory material has yet been obtained at present.
For improving the light fastness and the storage stability of the developed color image, it has been known to use of a mixture of two or more of chromogenic dye-precursors as described, for example, in Japanese Patent Publication No. 59-53193 (1984) (U.S. Pat. No. 4226912), Japanese patent application laying-open (KOKAI) Nos. 56-105990 (1981), 57-123085 (1982), 58-71192 (1983), 58-119892 (1983) and 61-76387 (1986). However, any mixture disclosed in these publications is prepared by mixing fluoran compounds with each other and thus it cannot always be said that the light fastness and the storage stability of developed color images are excellent.
The present inventors have made an earnest study for improving the foregoing drawbacks and, as a result, it have been found that the storage stability of the developed color image can be improved and the color developing sensitivity and the light fastness of the image can also be improved unexpectedly when a mixture prepared by mixing at least two divinyl phthalide compounds of the formula (I) or mixing the divinyl phthalide compound(s) of the formula (I) with at least one compound selected from the compounds represented by the formulas (II), (III) and (IV) is used as a chromogenic dye-precursor, and based on this findings, the present invention has been accomplished.